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Date: Fri, 27 Feb 2009 15:25:59 +1100
From: Paul Mackerras <paulus@samba.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Bharata B Rao <bharata.rao@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>,
       Paul Menage <menage@google.com>,
       Balbir Singh <balbir@linux.vnet.ibm.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] cpuacct: add a branch prediction
In-Reply-To: <20090226122623.GC23099@elte.hu>
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Ingo Molnar writes:

> * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > On Thu, 2009-02-26 at 21:06 +0900, KAMEZAWA Hiroyuki wrote:
> > > Hmm.. some routine like
> > > atomic64_read() can help this ? (But I don't want to use atomic_t here..)
> > 
> > Yeah, atomic64_t has been proposed numerous times, and x86 
> > could actually implement that using cmpxchg8b, just not sure 
> > about all the other 32bit archs, and if we start using it in 
> > the scheduler, they'd better have it implemented.
> 
> I have written a working atomic64_t implementation for 
> tip:perfcounters/core, for 32-bit x86.

atomic64_t would be a big problem for 32-bit powerpc.  We'd have to
use an array of spinlocks, or make atomic64_t actually be 12 bytes so
we have a lock word to use.

Paul.